The present invention relates to a method of preparing a dye solution, and to a silver halide emulsion, a silver halide photosensitive material and their respective production methods.
Hitherto, much effort has been expended to increase the sensitivities of silver halide photosensitive materials and to reduce color contamination after photographic processing. Photography has so far been required to ensure higher sensitivity and higher image quality on one hand, and on the other, recent years have seen strengthening requirements for photographic processing that the processing speed should be increased and the amounts of liquid wastes should be minimized from the viewpoint of environmental friendliness in particular. Therefore, the arts of spectrally sensitizing silver halide grains to higher degrees without any adverse effects, including fogging and color contamination, are of increasing importance.
Sensitizing dyes used for spectral sensitization are known to have great effect upon properties of silver halide photographic materials. It is already disclosed by us in JP-A-2001-312023 that high sensitivity and low color contamination are both achieved at the same time by the silver halide photographic material containing at least two sensitizing dyes having dissociable groups other than sulfonic acid group.
Each of the sensitizing dyes disclosed in the document cited above, though can be dispersed directly in emulsions, is generally dissolved in an appropriate solvent, such as methanol, ethanol, methyl cellosolve, acetone, water, pyridine or a mixture of two or more thereof, and then added to an emulsion. Therein, the dissolving operation may be carried out in the presence of an additive such as an acid, a base or a surfactant, or with the aid of ultrasonic waves. As to methods usable for adding such a compound, U.S. Pat. No. 3,469,987 discloses the method of dissolving the compound in a volatile organic solvent, dispersing the solution obtained into a hydrophilic colloid, and adding the resulting dispersion to an emulsion; JP-B-46-24183 discloses the method of dispersing the compound into a water-soluble solvent, and adding the dispersion obtained to an emulsion; U.S. Pat. No. 3,822,135 discloses the method of dissolving the compound in a surfactant, and adding the solution obtained to an emulsion; JP-A-51-74624 discloses the method of dissolving the compound with the aid of a red shift compound, and adding the solution obtained to an emulsion; and JP-A-50-80826 discloses the method of dissolving the compound in an acid free of water in a substantial sense, and adding the solution obtained to an emulsion. Further, the methods disclosed in U.S. Pat. Nos. 2,912,343, 3,342,605, 2,996,287 and 3,429,835 can be applied in adding the compounds to emulsions. Moreover, as disclosed in JP-A-7-140580, the compounds can take the form of a solid particulate dispersion in adding them to emulsions.
When sensitizing dyes have no counter ions, however, they are slightly soluble in generally used solvents, such as water and methanol. For solutions of such dyes to have practicable concentrations, it is required that bases such as sodium hydroxide and triethylamine are added in making the solutions and dissociate dissociable groups contained in the dyes. In the case where the dyes are dissolved completely according to the foregoing method, the solutions obtained have high pH of the order of 10-11. In such a high pH range, the dyes are apt to decompose. Therefore, silver halide emulsions to which the dye solutions prepared in accordance with the foregoing method are added are liable to suffer deterioration in photographic properties, such as a fog increase and soft gradation enhancement. On the other hand, lowering the pH of the dye solutions produces a dye precipitation problem.
Therefore, it has been strongly desired to find out methods of making solutions of sensitizing dyes having concentrations high enough to practical use under pH as close to neutral as possible.
Objects of the invention are to provide a fog-reduced silver halide photographic emulsion and a silver halide photographic material including the emulsion, and to provide a method of preparing a highly stable solution containing a high concentration of dye. This method is usable as a means in providing the emulsion and the photographic material.
As a result of our intensive studies, it has been found that the foregoing objects of the invention could be achieved by the embodiments (1) to (11) described below.
(1) A method of preparing a dye solution, which comprises dissolving a counter ion-free dye and a base having a pKa value of from 6.6 to 9.0 in a solvent having a relative dielectric constant of at least 10 (preferably, 90 or less) at 20xc2x0 C.
(2) The method of preparing a dye solution as described in the item (1) above, wherein the dye is a compound represented by the following formula (I):
Dyexe2x80x94((A),Q)qxe2x80x83xe2x80x83(I)
wherein Dye represents a dye moiety; A represents a linkage group; Q represents a dissociable group; r represents 0 or 1 and q represents an integer of 1 or more (preferably, 4 or less), provided that at least one Q is selected from the group consisting of xe2x80x94COOH, xe2x80x94CONHSO2R, xe2x80x94SO2NHCOR, xe2x80x94SO2NHSO2R and xe2x80x94CONHCOR and R represents an alkyl group, an aryl group, a heterocyclic group, an alkoxy group, an aryloxy group, a heterocyclyloxy group or an amino group.
(3) The method of preparing a dye solution as described in the item (1) or (2), wherein the dye is a compound represented by the following formula (II): 
wherein R1 and R2 each represent a substituted or unsubstituted alkyl, aryl or heterocyclic group, provided that at least one of R1 and R2 is substituted with xe2x80x94COOH, xe2x80x94CONHSO2R3, xe2x80x94SO2NHCOR3, xe2x80x94SO2NHSO2R3 or xe2x80x94CONHCOR3; R3 represents an alkyl group, an aryl group, a heterocyclic group, an alkoxy group, an aryloxy group, a heterocyclyloxy group or an amino group; Y1 and Y2 each represent atom group constituting a 5- or 6-membered nitrogen-containing heterocyclic ring which may be condensed together with a carbon ring or the other heterocyclic ring; V1 and V2 each represents a substituent; n1 and n2 each represent an integer of 0 or more (preferably 4 or less, more preferably 0 or 1), and when n1 and n2 each are 2 or more, V1s and V2s each may be the same or different; L1, L2, L3, L4, L5, L6 and L7 each represent a methine group; and p1 represents 0, 1, 2 or 3, and p2 and p3 each represent 0 or 1, and when p1 is 2 or 3, the repeated L2s and L3s each may be the same as or different from each other.
(4) The method of preparing a dye solution as described in any one of the items (1) to (3), wherein the pKa value of the base is from 7.0 to 8.0.
(5) The method of preparing a dye solution as described in anyone of the items (1) to (4), wherein the base is a compound represented by the following formula (III), (IV), (V) or (VI): 
wherein R11, R12 and R13 each represent a hydrogen atom, or a substituted or unsubstituted alkyl group, provided that at least one of R11, R12 and R13 is an alkyl group substituted with a radical having a "sgr"m value of 0 or more (preferably 0.8 or less); 
wherein R21, R22, R24 and R25 each represent a hydrogen atom or a substituent, and R24 and R25 may combine with each other to form a carbon ring or a heterocyclic ring; 
wherein R32, R33 R34, R35 and R36 each represent a hydrogen atom or a substituent; 
wherein R42, R43, R44, R45 and R46 each represent a hydrogen atom or a substituent, provided that at least two of R42, R43, R44, R45 and R46 are unsubstituted alkyl groups or alkyl groups substituted with radicals having "sgr"m values of 0 or less (preferably xe2x88x920.2 or more), and R42 and R43, R43 and R44, R44 and R45, or R45 and R46 may combine with each other to form a carbon ring or a heterocyclic ring.
(6) The method of preparing a dye solution as described in any one of the items (1) to (5), wherein the solvent is one of a solvent having a relative dielectric constant of at least 20 at 20xc2x0 C. and a mixture of two or more solvents having a relative dielectric constant of at least 20 at 20xc2x0 C.
(7) The method of preparing a dye solution as described in the item (6), wherein the solvent is one of water, methanol, and a mixture of water and methanol.
(8) A method of producing a silver halide emulsion, which comprises adding at least one of dye solutions prepared by the method described in any one of the items (1) to (7).
(9) A silver halide emulsion comprising at least one of dye solutions prepared by the method described in any one of the items (1) to (7).
(10) A method of producing a silver halide photographic material, which comprises coating on a support at least one silver halide emulsion layer containing the silver halide emulsion described in the item (9).
(11) A silver halide photographic material, comprising a support and at least one silver halide emulsion layer containing the silver halide emulsion described in the item (9).